An active matrix substrate used in a liquid crystal display device, or the like, includes a switching element such as a thin film transistor (hereinafter “TFT”) for each pixel. As such a switching element, a TFT using an oxide semiconductor layer as the active layer (hereinafter referred to as an “oxide semiconductor TFT”) is known in the art. Patent Document No. 1 discloses a liquid crystal display device using InGaZnO (an oxide composed of indium, gallium and zinc) in the active layer of the TFT.
Oxide semiconductor TFTs are capable of operating faster than amorphous silicon TFTs. Since oxide semiconductor films are formed by a simpler process than polycrystalline silicon films, it can be applied to devices that require large areas. Thus, oxide semiconductor TFTs have had high expectations as high-performance active elements that can be manufactured while suppressing the number of manufacturing steps and the manufacturing cost.
Since an oxide semiconductor has a high mobility, it is possible to realize a level of performance that is greater than or equal to that of an amorphous silicon TFT even when the element size is reduced relative to an amorphous silicon TFT. Thus, when an active matrix substrate of a display device is manufactured by using an oxide semiconductor TFT, it is possible to reduce the area of the TFT within a pixel, thereby improving the pixel aperture ratio. Thus, it is possible to produce bright display even when the amount of light of the backlight is reduced, thereby realizing a low power consumption.
Since oxide semiconductor TFTs have desirable off-leak characteristics, it is possible to use a driving method with which display is produced while reducing the image data rewrite frequency. For example, when displaying a still image, they can be driven so that the image data is rewritten once per second. Such a driving method is called pause drive or low-frequency drive, and it is possible to significantly reduce the power consumption of the display device.